Surface hardening treatment is treatment for generating residual stress on a surface of steel to improve abrasion resistance and fatigue resistance simultaneously with hardening the surface of the steel. Examples of method for typical surface hardening treatment that are currently in practical use include carburizing treatment and nitriding treatment.
The carburizing treatment is treatment involving increasing the temperature of the steel to a γ region and diffusing and permeating carbon over the surface of the steel. After the carburizing, quenching is performed to attempt surface hardening. Since the temperature of the steel is increased to the high temperature region in the carburizing treatment, deep hardening can be achieved. However, since it is necessary to perform quenching and tempering after the carburizing, strain is likely to be generated. Therefore, the steel subjected to the carburizing treatment cannot be used for the parts that are used for components accompanying rotation such as a transmission of an automobile. Although the strain can be removed by carrying out special treatment such as press-tempering treatment after the quenching, loss in time and cost accompanied by the special treatment cannot be avoided.
On the other hand, nitriding treatment is treatment involving diffusing and permeating nitrogen at temperature lower than or equal to an A1 point. Since the heating temperature in the nitriding treatment is 500 to 550° C., which is low, so that phase transformation does not occur due to heating, thus, no strain is generated in the steel while it is the case with the carburizing treatment. However, the time taken for the treatment is 50 to 100 hours, which is remarkably long, and it is also necessary to remove a brittle compound layer that has been generated on the surface after the treatment. Also in this case, loss in time and cost cannot be avoided.
Accordingly, there has been developed a method called soft-nitriding treatment. In the soft-nitriding treatment, the steel sheet is heated to temperature lower than or equal to the A1 transformation temperature, and nitrogen is diffused and permeated from the surface of the steel sheet. In this event, by using a carburizing atmosphere, carbon is also additionally diffused and permeated. Since no quenching is necessary as in the case with the carburizing treatment, no strain is generated due to the phase transformation. Further, since the treatment is carried out at relatively low temperature, thermal strain is small. Accordingly, the surface layer of the steel sheet can be hardened without deteriorating precision of a shape of a part. In addition, the time taken for the treatment is approximately half the time taken for the nitriding treatment. Therefore, the soft-nitriding treatment has rapidly been spread widely recently as a method of the surface hardening treatment for parts used in a mechanical structure.
Moreover, the soft-nitriding treatment is often carried out after performing press working to obtain a desired shape of the part. In particular, a part used in a mechanical structure such as a transmission part of an automobile is subjected to the press working from the viewpoint of productivity. Accordingly, a demand is increasing, for a steel sheet for soft-nitriding treatment having excellent moldability which is suitable for a material of a part used in a mechanical structure such as a transmission part of an automobile, and various techniques have been proposed so far.
For example, Patent Literature 1 discloses a method of manufacturing a steel nitride member having excellent cold forgeability and fatigue resistance, and Patent Literature 2 discloses a method of manufacturing a steel nitride member having small strain caused by heat treatment. Further, Patent Literatures 3 and 4 each disclose a steel sheet for nitriding having excellent moldability.
Patent Literature 5 discloses a steel for soft-nitriding treatment whose cost is low and which has satisfactory press workability. Further, Patent Literature 6 discloses a thin steel sheet for nitriding treatment which can obtain, after the nitriding treatment, high surface hardness and sufficient hardening depth. Still further, Patent Literature 7 discloses a steel sheet for soft-nitriding treatment having both excellent processability and fatigue resistance, and Patent Literature 8 discloses a steel sheet for soft-nitriding treatment having excellent moldability and strength stability after the soft-nitriding treatment.